Let's try to crunch the numbers on this one (all values are based on the Survival gamemode on PC version 1.055, obtained from The big ole doc of everything in the sidebar):

The base M9 Grenade Launcher deals 225 Explosive damage at the center of its blast radius and 150 Ballistic_Shell damage upon a direct impact regardless of projectile detonation. The humble Scrake has a x0.4 damage multiplier against explosive damage, and I presume Ballistic_Shell is unresisted across the board (with one specific exception that doesn't apply here) given my inability to dig up any more information about it. Let's also assume you're hitting him in the face, but far enough away that the projectile still detonates (good luck with that); that's a x1.1 hitzone multiplier, but only for the impact, because explosive damage doesn't account for hitzones.

We'll also assume you're a max-level Demolitionist with damage-maximizing skill choices because you're trying to kill a Scrake with the M79 and need all the help you can get. You'll have:

+25% damage from passive bonuses

+25% damage from Bombardier

+25% impact damage from High-Impact Rounds

and +50% impact damage to critical areas (hitzones with a damage multiplier greater than 1) from Armor Piercing Rounds

...which sums up to +50% Explosive damage and +125% Ballistic_Shell damage given the assumptions above. Now, to the actual damage calculations.

First, base weapon damage is calculated by a weapon-specific formula. This is usually only relevant for melee weapons, where damage increases or decreases based on the angle of the swing, so we'll just use the two values above.

Then, we apply the Scrake's damage resistances, also rounded down. Ballistic_Shell damage remains unchanged at 137 due to the aforementioned neutrality presumption, but Explosive damage is knocked down to (225 x 0.4) = 90. From here we would also take into account block modifiers, but you shouldn't be shooting at it while it's blocking anyway. No change.

Lastly, we apply perk-specific modifiers, which we round for Demolitionist. With the above perk setup, we get (137 + (137 x 1.25)) = 308 Ballistic_Shell damage and (90 + (90 x .5)) = 135 Explosive damage. Optionally, we can apply a fully-stacked Focus Injection buff (+20% damage, rounded up) courtesy of your local Medic, but remember that this won't be a feasible option if you're flying solo. The buff would give you (308 x 1.2) = 370 Ballistic_Shell damage and (135 x 1.2) = 162 Explosive damage. Past this, there is one more damage calculation stage if the hit is supposed to decapitate, but at this point its death is a foregone conclusion, so we'll ignore it.

In total, you are inflicting 443(532) body damage and 308(370) head damage per shot to the Scrake. Here's a rundown of damage thresholds, assuming the Scrake starts with full health and you are the only one inflicting damage, and only with the M79 (no distinctions will be made between instant death and "merely" decapitated; see above about foregone conclusions):

One headshot will never kill a Scrake from full under any circumstances.

Two headshots will kill up to 2p Normal and 1p Hard/Suicidal/Hell on Earth with the buff and 1p Normal/Hard without.

Three headshots will kill up to 5p Normal, 4p Hard, and 3p Suicidal/Hell on Earth with the buff; and 3p Normal and 2p Hard/Suicidal/Hell on Earth without.

Four headshots will kill up to 6p Normal/Hard and 5p Suicidal/Hell on Earth with the buff; and 6p Normal and 4p Hard/Suicidal/Hell on Earth without.

Five headshots will kill on all difficulties and playercounts with the buff and all difficulties and playercountsexcept6p Hell on Earth" without.

Six headshots is a guaranteed kill on all difficulties and playercounts, buff or not.

Bear in mind that these thresholds don't tell the whole story. Sometimes (but not all the time, reader beware), a damage threshold is barely missed, and decapitation can still be achieved with but a few shots from another weapon. In any case, the M79 should be relegated to strictly trash-clearing in higher level play, especially so since the Scrake's head will be flailing about and overall being a bothersome target after you get the first shot or two in.

Verify first that your issue isn't one of difficulty inflation due to health scaling from having more players on the server. Yes, it was stupidly difficult the first time it came around. We worked around it by playing in smaller parties, because specimen health regenerates at a fixed rate based on their maximum health (regular specimens recover from near-death to full health in ten seconds and Hans regenerates from near-death to full in 100, but the Patriarch may take either 50 or 100 seconds depending on how many players are still alive) and is always on. The more players you have on the server, the harder it is to outpace the health regen.

If it's still happening in parties of one to three players, then there would be a problem. You could go to the Tripwire forums to report it, in that case. But so far you haven't given us any information about your party composition when you experience this issue, so the default assumption is that it is working normally and your choice of perks and party is simply poor.

I have my doubts as to the range of Zelda's magic as you described. The Citadel was heavily implied to be the site of the royal army's final stand, if the advantageous defensive position and the cannons not found anywhere else in the game are anything to go by. It was certainly possible, if exceedingly difficult, for vanilla footsoldiers to take out or at least disable/immobilize Guardians with conventional weaponry, which would account for the ones found anywhere other than Fort Hateno, the Akkala Tech Lab, and maybe along the paths Link and Zelda traveled when the Calamity struck.

If I recall, there should be a Lizalfos just north of Mount Lanayru that is guaranteed to spawn with an Eightfold Longblade. Just something to keep in mind, as I don't usually see it among lists of limited-quantity equipment.

"Mad Scientist" weaponry was alluded to by the devs since before Early Access. My guess is that it refers to the less conventional and (optionally) more high-tech weapons already part of the game's repertoire. The Microwave Gun and Railgun fulfill both of these criteria already, though the more roughshod selections like the Pulverizer, Eviscerator, and Spitfires certainly qualify as well.

Given that it seems to be a straight liquid nitrogen hose, it is probable that achieving slowing-to-immobility would necessitate sustained fire, with greater quantities required for heavier threats (kinda like the cryo grenades pre-buff/against bosses, actually). Whether the gun's stream will be strictly single-target or have a splash effect is uncertain.

The "Ice" refers to the specific subcategory of low% runs in which the Ice Beam is collected as part of the bare minimum over, say, something like the Speed Booster. Zoasty had recently broken the world record in this category several times over separate streams, so there's quite a bit of research material if you're interested.

Lucky for you, the advanced Guardian Scouts are also among the creatures that respawn with every Blood Moon. They become an excellent source of anti-Guardian equipment, should you find yourself in need of such luxuries.